Sen. Bernie Sanders (D) of Vermont waves to the audience before speaking at the opening of the 2015 National Action Network Convention in New York City earlier this month. Senator Sanders is expected to announce his candidacy for president Thursday.

Mike Segar/Reuters

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Washington

The battle for the left has been joined. Sen. Bernie Sanders (I) of Vermont, a self-described “democratic socialist,” is becoming a Democrat to run for president, his advisers say.

Senator Sanders’s announcement, expected Thursday, sets up a David vs. Goliath scenario against former Secretary of State Hillary Rodham Clinton, who is strongly favored to win the Democratic nomination in 2016. But for now, it’s the debate that Sanders may fuel, not the outcome of the primaries, that matters.

Sanders plans to focus on three core issues – income inequality, campaign finance reform, and climate change – informal adviser Tad Devine tells Politico. And by entering the race, he threatens to nudge Mrs. Clinton further to the left than she might otherwise go.

By becoming a Democrat, after a long political career as an independent, Sanders is positioning himself to face Clinton on stage in the party’s primary debates. (As an independent, he would be ineligible.)

But already, in fact, the Sanders-Warren wing of the Democratic Party seems to have had an effect on Clinton. In her announcement video and after, she has struck a more populist tone than she did in her first presidential campaign, in 2008.

Clinton calls herself a champion for “everyday” Americans, and speaks of how “the deck is still stacked in favor of those at the top.” In an e-mail to supporters the morning she announced, she said, "There's something wrong when CEOs make 300 times more than the typical worker.”

Given the controversy over donations to Clinton’s family foundation, plus the large sums she is collecting for her campaign with the help of super-wealthy donors, it’s unclear how much she will be able to focus on the “evils” of big money. It’s also unclear how much Sanders will go after Clinton and her vulnerabilities over money.

Sanders himself plans not to go for the big campaign cash that has become a hallmark of modern campaigns, and instead will solicit small-dollar donations, according to reports.

Sanders also doesn’t want to be a spoiler in the general election.

“The one thing he’s determined not to do is to be another Ralph Nader. And the only way to avoid doing that is to avoid being a third-party candidate from the left in the general election,” Mr. Devine tells Politico, noting that Sanders has long caucused with the Democrats.

Today, the great unknown is how big a force the feisty Sanders, a native of Brooklyn, N.Y., will become on the Democratic left. Unlike former Rhode Island Gov. Lincoln Chafee, a onetime Republican who has failed to attract much attention after entering the Democratic nomination race earlier this month, Sanders already has a national following.

“With his trademark sarcasm, he mocked the rich, yelling: ‘How can I get by on one house? I need five houses, 10 houses! I need three jet planes to take me all over the world!’” the AP reports. “The speech was so popular it crashed the Senate video server. It was later printed in a small book.”

According to advisers, Sanders’s announcement Thursday will be low key, followed by a larger kickoff rally, likely in Vermont. Sanders served as mayor of Burlington, Vt., for most of the 1980s, followed by 16 years in the House. He was elected to the Senate in 2006.